The present invention relates to an overcurrent detection circuit, and more specifically, to an overcurrent detection circuit for use in a power supply circuit for a DC-DC converter or the like.
In the prior art, a power supply circuit for a DC-DC converter or the like drives an output transistor to control current that is supplied to a load. Normally, a power supply circuit includes an overcurrent detection circuit to detect overcurrent flowing to the output transistor and protect the power supply circuit from the overcurrent.
Japanese Laid-Open Patent Publication No. 2003-224968 describes a differential amplifier 1 that detects overcurrent flowing to a switching element M1 (output transistor) based on a reference voltage source V1 (see FIG. 1 of the publication). Japanese Laid-Open Patent Publication No. 6-311734 discloses a comparator 56 for detecting overcurrent flowing to a switch element M14 (output transistor) by comparing an output voltage of a current detector 49 with a reference voltage Vref (refer to FIG. 1 of the publication).
However, the prior art circuits do not take into consideration on-resistance variations of an output transistor caused by temperature or manufacturing process factors. In Japanese Laid-Open Patent Publication No. 2003-224968, the differential amplifier 1 cannot accurately detect the overcurrent flowing to the switching element M1 if the on-resistance of the switching element M1 deviates from a designated value due to temperature or manufacturing process factors. Similarly, in Japanese Laid-Open Patent Publication No. 6-311734, the comparator 56 may not be able to accurately detect overcurrent due to variations in the on-resistance of the switch element M14.
Accordingly, Japanese Laid-Open Patent Publication No. 2005-333691 discloses an overcurrent detection circuit 14 for detecting the overcurrent irrespective of variations in the on-resistance of a power MOS transistor 2, which functions as an output transistor (refer to FIG. 1 of the publication). The overcurrent detection circuit 14 generates a reference current that is dependent on the on-resistance of the power MOS transistor 2 with a detection MOS transistor 3, which is formed on the same chip as the power MOS transistor 2, and a constant current circuit 4, which is connected to the detection MOS transistor 3. This prevents detection errors that would be caused by variations in the on-resistance of the power MOS transistor 2.
However, in the circuit disclosed in Japanese Laid-Open Patent Publication No. 2005-333691, a reference current generation circuit (detection MOS transistor 3 and constant current circuit 4) is always detecting overcurrent value. In a power supply circuit incorporating a plurality of DC-DC converters that generate different power supplies as in a multi-channel DC-DC converter, each DC-DC converter requires a reference current generation circuit that is in correspondence with the overcurrent value that is to be detected. This increases power consumption and enlarges the chip area. In the circuit disclosed in Japanese Laid-Open Patent Publication No. 2005-333691, only one level of overcurrent value can be detected. Thus, a single reference current generation circuit cannot detect multiple overcurrent levels.